Add worktree subcommand

[pjbgf]

Adds a new worktree command with sub commands: add, remove and list.

The output of list was aligned with upstream for the supported cases (e.g. we currently don't support prune):

go-git/cli

$ ./build/gogit worktree list
/src/git/go-git/cli      a411c9b [worktrees]
/tmp/abc                       a411c9b (detached HEAD)
/tmp/abc1                      a411c9b [abc1]
/tmp/abc3                      f4f6863 (detached HEAD)
/tmp/abc4                      f4f6863 [abc4]
/tmp/abc5                      f4f6863 (detached HEAD)
/tmp/abc6                      a411c9b (detached HEAD)

upstream git

$ git worktree list
/src/git/go-git/cli  a411c9b [worktrees]
/tmp/abc                   a411c9b (detached HEAD)
/tmp/abc1                  a411c9b [abc1]
/tmp/abc3                  f4f6863 (detached HEAD)
/tmp/abc4                  f4f6863 [abc4]
/tmp/abc5                  f4f6863 (detached HEAD)
/tmp/abc6                  a411c9b (detached HEAD)

This command was largely made to test the go-git linked worktree implementation.

[Copilot]

Pull request overview

This PR adds a new worktree command to the go-git CLI that enables management of Git worktrees with three subcommands: add, list, and remove. The implementation aims to align output formatting with upstream Git for the list command and supports testing the go-git linked worktree functionality.

• Adds comprehensive worktree management capabilities (add, list, remove)
• Updates go-git and related dependencies to versions with worktree support
• Aligns list command output format with upstream Git

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 7 comments.

File	Description
cmd/gogit/worktree.go	New file implementing worktree subcommands with add, list, and remove functionality
go.mod	Updates go-git, go-billy, and go-git-fixtures dependencies to versions supporting worktree features
go.sum	Corresponding checksum updates for the new dependency versions

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